Method for producing semiconductor device

ABSTRACT

An amorphous silicon film is formed on a flat glass substrate, and then crystallized by heating to obtain a crystalline silicon film. The glass substrate is placed on a stage having a convex U-shaped curved surface. The glass substrate is heated for a desired period of time at a temperature close to a strain point of the glass substrate, and then is cooled. Also, an amorphous silicon film formed on a glass substrate is crystallized into a crystalline silicon film by heating and then the glass substrate is mounted on a stage having a flat surface in such a manner that the lower surface of the glass substrate is in close contact with the flat surface of the stage by pressing the upper surface of the glass substrate. Then, a linear laser beam is irradiated on the crystalline silicon film in a scanning manner.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and an apparatus for producinga semiconductor device which is capable of obtaining a crystallinesilicon film high in uniformity by improving the flatness of a glasssubstrate during producing an insulated gate semiconductor device suchas a thin film transistor (TFT) formed using a non-single crystalsilicon film which is formed on the substrate, or other semiconductordevices. In particularly, the present invention is useful in producing asemiconductor device formed on the glass substrate.

2. Description of the Related Art

In recent years, several researches have been made into insulated gatefield effect transistors having a thin film-shaped active layer(so-called active region) on an insulated substrate, which are so-calledTFT).

Those TFTs are classified, for example, into an amorphous silicon TFTand a crystalline silicon TFT, depending upon the material or thecrystal state of a semiconductor as used. The crystalline silicon statedhere is directed to non-single crystal silicon which is not singlecrystal. Hence, those TFTs are generally called "non-single crystalsilicon TFTs".

In general, the electric field mobility of a semiconductor which is inan amorphous state is small, and therefore not available to the TFTsthat require a high speed operation. Also, the amorphous silicon cannotbe used to produce a p-channel TFT (PMOS TFT) since the p-type electricfield mobility of the amorphous silicon is remarkably small, and thus acomplementary MOS circuit (CMOS) cannot be formed by the combination ofthe p-channel TFT and an n-channel TFT (NMOS TFT).

The crystalline semiconductor is larger in the electric field mobilitythan the amorphous semiconductor, and therefore enables a high speedoperation. The crystalline silicon can be used for obtaining not onlythe NMOS TFT but also the PMOS TFT, thereby forming a CMOS circuit.

A crystalline silicon film is obtained by thermally cooling an amorphoussilicon film obtained through the vapor phase growth technique at anappropriate temperature (600° C. or higher) for a long period of time,or by irradiating an intense light such as a laser beam (opticallyannealing).

However, in the event of using a glass substrate which is inexpensiveand rich in processability as an insulating substrate, it is extremelydifficult to obtain, by only annealing, a crystalline silicon film whichis satisfactorily high in electric field mobility (high to the degreethat the CMOS circuit can be formed).

This is because the above glass substrate is generally low in strainpoint (about 600° C.), and thus when the temperature of the substrate iselevated up to a temperature required for obtaining a crystallinesilicon film sufficiently high in mobility, the substrate is warped.

In the event of applying the optically annealing technique forcrystallizing a silicon film formed on the glass substrate, a highenergy can be applied only to the silicon film without elevating thetemperature of the substrate so much. Hence, the optically annealingtechnique is very effective in crystallizing the silicon film formed onthe glass substrate.

It has been found that a high power pulse laser such as an excimer laseris the most suitable for the optimum light source for opticallyannealing. The maximum energy of the laser is extremely larger than thatof the continuously oscillating laser such as an argon ion laser, andthe mass productivity could be enhanced using a large spot which isseveral cm² or more in size.

However, because the beam as normally used is square or rectangular inshape, the beam is required to move vertically and horizontally forprocessing a single substrate having a large area. Thus, the opticallyannealing technique needed to be still improved from the viewpoint ofthe productivity.

The above matter could be remarkably improved with a technique in whichthe beam is deformed linearly, the width of the beam is a length thatexceeds that of a substrate to be processed, and the substrate isscanned relatively by the beam (The scanning operation in thespecification means that linear laser beams are irradiated onto thesubstrate while being overlapped one on another bit by bit.). Thedetails are disclosed in Japanese Patent Unexamined Publication No.5-112355.

The silicon film which is still high in crystallinity can be prepared byconducting the thermally annealing process prior to conducting theoptically annealing process. In the thermally annealing process asdescribed in Japanese Patent Unexamined Publication No. 6-244204,utilizing the effect that an element such as nickel, iron, cobalt,platinum, paradium (hereinafter referred to as "crystallized catalyticelement, or simply "catalytic element") promotes the crystallization ofamorphous silicon, the crystalline silicon film can be obtained by thethermally annealing process at a lower temperature for a shorter periodof time in comparison with the normal case.

TFTs arranged in the form of a matrix are formed with a crystallinesilicon film formed using the thermally annealing process and theoptically annealing process together, and the distribution of theirthreshold voltage in the substrate surface is investigated.

FIG. 2 shows the distribution of the threshold values of the TFT usingthe crystalline silicon film formed through the conventional method,within the substrate surface. The distribution is U-shaped as shown inFIG. 2.

FIG. 4 shows the arrangement of TFTs on the glass substrate. The data inFIG. 2 is obtained, as shown in FIG. 4, in such a manner that the TFTsof 400×300 pieces are arranged in the form of a matrix in a region of40×50 mm on a Corning 1737 substrate of 100 mm², and the respectivelocations of 400 TFTs disposed laterally in a line from one end to theother end (a portion surrounded by a dotted line in FIG. 4) areindicated correspondingly in the axis of abscissa.

When the pixel matrix that constitutes the pixel portion of a liquidcrystal display has the distribution of threshold voltages shown in FIG.2, the display state becomes nonuniform, resulting in a defective image.

As a result of researching the cause that the threshold voltage exhibitssuch a U-shaped distribution within the substrate surface by theapplicant, it has been found that a tendency of the U-shapeddistribution is very similar to the warp of the substrate immediatelybefore a laser beam is irradiated onto the substrate.

Also, no warp of the substrate is found in the glass substrateimmediately after an amorphous silicon film is formed on the glasssubstrate, and it has been found that the warp of the substrate iscaused because, during a heat treatment (by which the film grows in thesolid phase into a crystallized film) subsequent to the amorphoussilicon film forming process, a silicon film (or silicon oxide film) iscontracted higher than that of the glass substrate in cooling thesubstrate after the heat treatment. The warp of the substrate isproduced in a U-shape viewed from the film formed on the substrate.

FIG. 3 shows a state in which a laser annealing is conducted on thesilicon film formed on the substrate which has been warped. From FIG. 3,when the laser annealing is conducted on the silicon film in such astate where the substrate is warped, the focal point of the laser beamis shifted differently at the respective locations on the substrate.

It is presumed that the shift of the focal point makes the crystallinityof the silicon film different from each other within the substratesurface, so that the threshold voltage exhibits a specified distributionwithin the substrate surface.

The warp of the substrate immediately before a laser beam is irradiatedonto the substrate having 100 mm square is different by about 50 μmbetween the central portion and the edge portion of the substrate. Thedegree of the warp fell within a range of about 20 to 200 μm although itdepends upon the temperature of the above heat treatment process, a timenecessary for processing, the material of the substrate, or the like.There is a case in which when the size of the substrate is about 500 mmsquare, its warp becomes about 1 to 2 mm.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstances,and therefore an object of the present invention is to enhance theflatness of a substrate after the heating process and the annealingprocess have been conducted on the substrate on which a film is formed.

Another object of the present invention is to provide a method ofproducing a crystalline silicon film formed on a glass substrate andhaving a uniform crystallinity within a substrate surface. Anotherobject of the present invention is to provide a method of producing aplurality of crystalline silicon TFTs formed on a glass substrate andhaving a uniform threshold voltage within a substrate surface. Anotherobject of the present invention is to provide a method of producing, ina process of crystallizing a silicon film on a glass substrate, inparticular, having a thermally annealing process and a laser annealingprocess subsequent to the thermally annealing process, a crystallinesilicon TFT having a uniform crystallinity within a substrate surfaceand also uniform threshold voltage within the substrate surface usingthe silicon film.

In order to solve the above problems, the present invention is achievedby a method of producing a semiconductor, including the steps of formingan amorphous silicon film on a glass substrate or on a silicon oxidefilm formed on the glass substrate, flattening the glass substrate at atemperature equal to or higher than a strain point of the glasssubstrate and equal to or lower than a softening point thereof, andconducting a laser annealing process on the silicon film.

Also, the present invention is achieved by a method of producing asemiconductor, including the steps of forming an amorphous silicon filmon a glass substrate or on a silicon oxide film formed on the glasssubstrate, flattening the glass substrate at a temperature equal to orhigher than a strain point of the glass substrate and equal to or lowerthan a softening point thereof and crystallizing the amorphous siliconfilm, and conducting a laser annealing process on the silicon film whichhas been crystallized through the above process.

The present invention is achieved by a method of producing asemiconductor device, including a step of forming a plurality of thinfilm transistors (TFTs) having the silicon film produced by the abovesemiconductor producing method as an active layer.

As described above, in the process of producing a TFT formed on theglass substrate, the glass substrate is warped and deformed after thestep of thermally annealing the amorphous silicon film on the glasssubstrate.

Upon irradiating a laser beam onto the substrate thus deformed, theeffect of irradiation of the laser beam is different at the respectivelocations of the substrate.

Therefore, a laser beam is irradiated onto the substrate after thesubstrate is processed into an extreme flat state prior to the laserirradiating step.

The present invention has been made by irradiating a laser beam onto thesubstrate after the substrate before being subjected to the laserirradiating step is subjected to a thermal treatment into an extremeflat state.

The glass substrate on which the amorphous silicon film has been formedis thermally annealed at a temperature equal to or higher than thestrain point (593° C.) but equal to or lower than the softening point(844° C.) of the material of the glass substrate (for example, Corning7059), for example, at 640° C. for about four hours. Thus, in the eventof subjecting the substrate to the heat treatment in advance, theeffective method is that the substrate is held at a temperature equal toor higher than the strain point of the glass as used and equal to orlower than the softening point thereof for several hours, and thereafterthe substrate is cooled, from the viewpoint of the applicant'sexperiment. (It is difficult to flatten the substrate because it is sohard when the temperature is less than the strain point. The substrateis softened to the degree that the thickness of the substrate is changedwhen the temperature is higher than the softening point.)

The temperature within the temperature range, preferably a temperatureclose to an annealing point (639° C.) is most preferable in flatteningthe substrate.

In this case, the glass substrate is located on a base having a surfacewhich has been flattened with high accuracy (preferably the roughnessand waviness of the surface is 5 μm or less).

The glass substrate which is thermally annealed under the abovecondition is extremely lower in viscosity than that in the state of aroom temperature, and the glass substrate is brought in close contactwith the above base which is flattened with high accuracy byself-weight.

With cooling from the close contact state, the glass substrate issolidified while keeping that state. The glass substrate is flattenedwith high accuracy.

Also, the amorphous silicon film formed on the glass substrate isthermally annealed simultaneously in the above glass substrateflattening step so that the film grows in solid phase. Hence, thecrystallization of the silicon film can be conducted simultaneously whenthe glass substrate is flattened.

As a result that the applicant investigated the influence of any stepsfor forming the TFT on the substrate on the shape of the substrate, thedeformation of the substrate after the step of thermally annealing(including the cooling process) the amorphous silicon film has beencompleted is most remarkable, and no remarkable deformation has beenfound in the steps subsequent to the thermally annealing step. Hence, ifthe substrate is processed into a remarkable flat state immediatelybefore the irradiation of a laser beam onto the substrate, the substrateafter being subjected to all the steps can be kept in the flat state.

When a liquid crystal display is formed using the substrate, the glasssubstrate can be flattened extremely excellently, resulting in such anadvantage that the cell pair can be made readily and surely.

In general, if the roughness and waviness of the surface of thesubstrate constituting the liquid crystal display is out of 5 μm orless, something interferes with the cell pair. Hence, it is extremelyeffective that the roughness and waviness of the flattened base withhigh accuracy to be used in the present invention, as well as theroughness and waviness of the surface of the substrate as formed are 5μm or less.

Further, in order to solve the above problem, the present invention ischaracterized in that a base on which the glass substrate is mounted hasa convex curved surface. In other words, the present invention has beenachieved by a method of producing a semiconductor, characterized bycrystallizing an amorphous silicon film formed on a flat glass substrateby heating, locating the glass substrate on a base having the convexcurved surface, heating the glass substrate at a temperature close tothe strain point of the glass substrate, and cooling the glasssubstrate.

Also, the present invention has been achieved by a method of producing asemiconductor, including the steps of crystallizing an amorphous siliconfilm formed on a flat glass substrate by heating, locating the glasssubstrate on a base having the convex curved surface, heating the glasssubstrate at a temperature close to the strain point of the glasssubstrate, cooling the glass substrate, and thereafter irradiating alaser beam onto the silicon film.

As described above, in the process of producing the TFT, etc., formed onthe glass substrate, the glass substrate is warped and deformed afterthe step of thermally annealing the amorphous silicon film on the glasssubstrate.

Upon irradiation of a laser beam onto a substrate thus warped anddeformed, the focal point of the laser beam is different at therespective locations of the substrate, with the result that thecrystallinity becomes uniform within the substrate surface.

In view of the above, the glass substrate is made in a flat state afterthe thermally annealing step, and thus a laser beam is irradiated ontothe substrate, thereby crystallizing the substrate uniformly within thesubstrate surface.

FIGS. 6A to 6C show a producing method in accordance with the presentinvention. A glass substrate 601 which is deformed in a convex shape islocated on a base 602 (stage) having a convex curved surface which issubstantially symmetric with the curved surface of the substrate 601(deformed in the convex shape) which has been thermally annealed(thermally annealing and cooling) after a silicon film 600 is formed onthe substrate 601. The substrate 601 is deformed by heating at atemperature close to the strain point of the glass substrate so that itis brought in close contact with the convex curved surface of the base602 in FIG. 6B. Then, the substrate 601 is cooled. In cooling thesubstrate 601, a silicon film 600 contracts more markedly than that ofthe substrate 601, with the result that the substrate 601 changes fromthe convex curved surface state to the flat state as shown in FIG. 6C.

The present invention is achieved by a method of producing asemiconductor, including the steps of locating on a base having a convexcurved surface a flat glass substrate on which an amorphous silicon filmis formed, heating the glass substrate at a temperature close to thestrain point of the glass substrate, thereafter cooling the glasssubstrate.

The present invention is achieved by a method of producing asemiconductor, including the steps of locating on a base having a convexcurved surface a flat glass substrate on which an amorphous silicon filmis formed, heating the glass substrate at a temperature close to thestrain point of the glass substrate, cooling the glass substrate, andthen irradiating a laser beam onto the silicon film.

FIGS. 7A to 7C show a producing method in accordance with the presentinvention. In crystallizing an amorphous silicon film 700 through thethermally annealing step, a glass substrate 701 is mounted on a convexcurved surface type base 702 (stage) as shown in FIG. 7A, and thesubstrate is so heated as to be deformed into a convex curved surfacetype.

Then, the glass substrate 701 is deformed along the convex surface ofthe base 702 because of the lowering of the viscosity due to heating andthe self-weight. Keeping this state, the substrate is heated, and thencooled after the completion of the heat treatment.

In this state, the silicon film 700 contracts more sharply than that ofthe glass substrate 701, and the glass substrate 701 returns from theconvex curved surface type to the flat state (FIG. 7C).

In this way, the flattening of the glass substrate 701 and thecrystallization of the semiconductor film 700 can be conductedsimultaneously.

When the temperature necessary for the above glass substrate flatteningprocess is within 70 to 115% of the strain point of the substrate, theeffect of flattening the substrate is obtained.

When the heating temperature is lower than 70% of the strain point ofthe substrate, the substrate is not deformed at all, or a very long timeis required for the deformation of the substrate. When the heatingtemperature is more markedly than 115% of the strain point of thesubstrate, the deformation of the substrate is remarkable, and thus theshape of the substrate is not fixed after cooling.

In the case where the glass substrate is flattened simultaneously whenthe amorphous silicon film is crystallized, it is more preferable thatthe temperature is higher in order to enhance the crystallinity.However, it has been recognized that the crystallinity is sufficientlyimproved even within the above temperature range. The temperature rangeis a value in a case of setting an absolute zero point as a reference.

FIG. 8 shows the producing method in accordance with the presentinvention. A glass substrate 801 on which a silicon film has been formedis located along a base 802 (stage) having a convex curved surface bypressing the edges of the substrate 801 by pushers 803, or the like, andthe glass substrate 801 is deformed along the convex curved surface in astate before heating is conducted.

It is preferable that the stage 802 is made of quartz in order toprevent the substrate 801 from being tainted.

The glass substrate 801 is heated while maintaining this state, and thesilicon film formed on the glass substrate 801 is subjected to the laserannealing process in this state.

The heating temperature at this time is the room temperature to 70% ofthe strain point of the glass substrate.

When the heating temperature exceeds 70% of the strain point of theglass substrate 801, the glass substrate 801 is liable to be thermallydeformed, which makes it difficult to allow the substrate 801 to returnto the flat surface after cooling. In the case where the heatingtemperature is an excessively low temperature, that is, lower than theroom temperature, the crystallization becomes insufficient because heatis radiated.

Then, the substrate is cooled. In cooling the substrate, the siliconfilm contracts more sharply than that of the glass substrate 801, withthe result that the glass substrate 801 returns from the convex curvedsurface type to the flat state.

FIG. 9 shows a substrate heating unit. When the substrate is heated in asystem of FIG. 9, heating can be efficiently conducted on a substrate901 having a curved surface. A base 903 having a heater 902 is locatedunder the substrate 901, helium gas is heated by the heater 902, and thehelium gas thus heated is circulated under the substrate 901 by a pump904, thereby maintaining the substrate 901 to a desired temperature. Thesubstrate 901 is fixed by pushers 905. The helium gas is used becausethe thermal conductivity is high.

In FIG. 10, a glass substrate 951 on which a silicon film has beenformed is pushed on a base 952 (stage) having a convex U-shaped curvedsurface by pushers 953, thereby making the glass substrate 951 curveinto a convex U-shaped curved surface.

The glass substrate 951 is heated while maintaining this state, and thesilicon film formed on the glass substrate is annealed by a laser beamin this state.

The heating temperature at this time is the room temperature to 70% ofthe strain point of the glass substrate 951. A preferable heating methodis shown in FIG. 9.

When the heating temperature exceeds 70% of the strain point of theglass substrate 951, the glass substrate 951 is liable to be thermallydeformed, which makes it difficult to allow the substrate 951 to returnto the flat surface after cooling. In the case where the heatingtemperature is an excessively low temperature, that is, lower than theroom temperature, the crystallization becomes insufficient because heatis radiated.

A laser beam used for laser annealing is processed into a linear shapefor the purpose of enhancing the efficiency of a laser processing.

FIG. 11 shows a laser irradiating method. In FIG. 11, the height of abase (stage) indicated by a dotted line fluctuates in accordance withthe degree of a curvature of the substrate 960 in such a manner that thefocal surface of a laser beam is always positioned on a surface to beprocessed.

Since the degree of the curvature of the substrate 960 is found by theshape of the base, the thickness of the substrate, etc., in advance, theheight of the base is allowed to fluctuate on the basis of the data,whereby the focal surface of the linear laser beam may be kept constantregardless of the degree of curvature of the substrate. Also, an opticalsystem is not changed as it is, and under the substantially samecondition as in the case of using a flat substrate, the laser annealingcan be conducted.

In order to irradiate the linear laser beam onto the curved surfacewhich is curved into a U-shaped type as shown in FIG. 10, a laser beamis irradiated thereon as shown in FIG. 11, thereby being capable ofconducting a uniform laser irradiation regardless of the substrate beingcurved, to thus obtain a high processing efficiency and a highuniformity of laser annealing as in the flat substrate.

This is a case where a linear laser beam is irradiated onto the U-shapedcurved surface. Also, in the case of conducting a laser irradiation onthe convex curved surface using a laser beam which is not linear butsquare, the laser anneal can be conducted likewise.

The focal point of the laser beam may fluctuate by not the height of thesubstrate but the adjustment of a lens. However, to make the focal pointof the laser beam fluctuate, there is a case in which there is requiredsuch an optical design that the distribution of energy of the laser beamon the surface to be irradiated, the depth of focus thereof, etc., isnot changed.

Then, the substrate is cooled. In cooling the substrate, the siliconfilm contracts more markedly than that of the glass substrate, with theresult that the glass substrate returns from the convex curved surfacetype to the flat state, thereby obtaining a flat substrate having acrystalline silicon film.

As a result that the applicant investigated the influence of any stepfor forming the TFT on the substrate on the shape of the substrate, thedeformation of the substrate before and after the step of the heattreatment for crystallizing the silicon film is most remarkable, and noremarkable deformation has been found in the steps subsequent to theheat treatment. Hence, if the substrate is processed into a remarkableflat state immediately before the irradiation of a laser beam onto thesubstrate, the substrate after being subjected to all the steps can bekept in a fairly flat state.

The producing method of the present invention provides a crystallinesilicon film having an extremely uniform crystallinity within thesubstrate surface, and also provides a flat substrate.

In the present invention, the roughness and waviness of the glasssubstrate can be about 10 μm or less in a substrate 1.1 mm in thicknessand 100 mm×100 mm in size. When the substrate is about 500 mm in size(for example, 370×400 mm², 400×500 mm², 550×650 mm² in size) and about0.5 to 0.7 mm in thickness, the degree of the warp of the substrateafter thermally crystallizing and cooling the amorphous silicon film maybe 1 to 2 mm in difference in level. However, the present invention canmanufacture the substantially flat substrate.

It should be noted that the convex curved surface and the U-shapedcurved surface of the base on which the glass substrate is mounted isdetermined in accordance with the size, the thickness and the materialof the glass substrate, the sort and the thickness of the film formed onthe substrate, and other various conditions.

As the substrate is increased in area, the degree of curvature of thesubstrate is increased more. Also, it is curved two-dimensionally.Hence, in case of the glass substrate of about 100 mm×100 mm, the baseon which the substrate is mounted may be of the shape having a U-shapedconvex curved surface which is curved in only one direction. In thiscase, it is desirable that the convex curved surface of the inverseU-shape type of the base has 20 to 200 μm, preferably about 50 μm in adifference in level between the central portion of a region of theconvex curved surface on which the glass substrate is mounted and thelowest portion of the edge of that region.

Also, When the size of the substrate is increased to the degree of 500mm square, the glass substrate may be curved in two directions.Therefore, it is preferable to use the base having the convex curvedsurface such that the sections along the two directions become theinverse U-shape type. In the case of using a large-area glass substrate,it is desirable that a difference in level between the central portionof a region of the convex curved surface on which the glass substrate ismounted and the lowest portion of the edge of that region is about 1 to2 mm.

As a result of forming a plurality of TFTs using a crystalline siliconfilm formed in accordance with the producing method of the presentinvention, the distribution of the threshold voltage of the TFTs can bemade extremely uniform within the substrate surface. This effect isincreased more as the substrate becomes large in area.

Also, the crystalline silicon TFTs for pixels and drive are disposed onthe glass substrate in accordance with the present invention, and aliquid crystal display is formed using the substrate. In this case,because the glass substrate can be flattened extremely excellently inaccordance with the producing method of the present invention, there isadvantageous in that the cell pair can be made readily and surely. Inthis case, even when there is no crystallizing step due to the laserirradiation after the thermal crystallization, the effect of theinvention to flatten the substrate is effective.

The present invention is characterized in that, in order to correct thewarp of the glass substrate flatly, the glass substrate is sucked on aflat surface of a stage having the flat surface, the peripheral portionof the glass substrate is pressed (pressurized), or the like. Also, thepresent invention is characterized in that such a flattening andcorrecting means is added to the stage of the laser annealing unit.

According to the present invention, there is provided a laser annealingmethod including the steps of flattening a glass substrate on a stageand mounting the former on the latter, and irradiating a linear laserbeam on the surface to be irradiated on the glass substrate in ascanning manner to conduct the laser annealing.

According to the present invention, there is provided a laser annealingmethod including the steps of crystallizing an amorphous silicon filmformed on a glass substrate by heating into a crystalline silicon film,flattening the glass substrate on the stage and mounting the former onthe latter, and irradiating a linear laser beam on the crystallinesilicon film in a scanning manner to conduct the laser annealing.

According to the present invention, there is provided a laser annealingmethod including the steps of crystallizing an amorphous silicon filmformed on a glass substrate by heating into a crystalline silicon film,mounting the glass substrate on a stage having a flat surface in such amanner that the lower surface of the glass substrate is in close contactwith the flat surface of the stage, and irradiating a linear laser beamon the crystalline silicon film in a scanning manner to conduct thelaser annealing.

According to the present invention, there is provided a laser annealingmethod including the steps of crystallizing an amorphous silicon filmformed on a glass substrate by heating into a crystalline silicon film,mounting the glass substrate on a stage having a flat surface in such amanner that the lower surface of the glass substrate is sucked with theflat surface of the stage under vapor, and irradiating a linear laserbeam on the crystalline silicon film in a scanning manner to conduct thelaser annealing.

According to the present invention, there is provided a laser annealingmethod including the steps of crystallizing an amorphous silicon filmformed on a glass substrate by heating into a crystalline silicon film,mounting the glass substrate on a stage having a flat surface in such amanner that the lower surface of the glass substrate is in close contactwith the flat surface of the stage by pressing the peripheral portion ofthe upper surface of the glass substrate, and irradiating a linear laserbeam on the crystalline silicon film in a scanning manner to conduct thelaser annealing.

According to the present invention, there is provided a laser annealingdevice including a stage having means for flattening and mounting aglass substrate, and means for irradiating a linear laser beam onto asurface to be irradiated of the glass substrate in a scanning manner.

According to the present invention, there is provided a laser annealingdevice including a stage having a flat surface on which a glasssubstrate is mounted and means for contacting the lower surface of theglass substrate with the flat surface thereof, and means for irradiatinga linear laser beam onto a surface to be irradiated of the glasssubstrate in a scanning manner.

According to the present invention, there is provided a laser annealingdevice including a stage having means for flattening and mounting aglass substrate having thereon a crystalline silicon film which has beencrystallized by heating, and means for irradiating a linear laser beamon a crystalline silicon film on the glass substrate in a scanningmanner.

According to the present invention, there is provided a laser annealingdevice including a stage having a flat surface on which a glasssubstrate having a crystalline silicon film crystallized by heating ismounted and means for contacting the lower surface of the glasssubstrate with the flat surface thereof, and means for irradiating alinear laser beam onto the crystalline silicon film in a scanningmanner.

According to the present invention, there is provided a laser annealingdevice including a stage having a flat surface on which a glasssubstrate having a crystalline silicon film crystallized thereon byheating is mounted and means for making the lower surface of the glasssubstrate suck on the flat surface thereof under vapor, and means forirradiating a linear laser beam onto the crystalline silicon film in ascanning manner.

According to the present invention, there is provided a laser annealingdevice including a stage having a flat surface on which a glasssubstrate having a crystalline silicon film crystallized thereon byheating is mounted and means for pressing the peripheral portion of theupper surface of the glass substrate, and means for irradiating a linearlaser beam onto the crystalline silicon film in a scanning manner.

According to the present invention, in the above structure, thecrystalline silicon film may be a film a part of which contains animpurity by ion doping or the like.

Also, according to the present invention, in the above structure, apulse laser, more preferably an excimer laser may be used as a lightsource of the linear laser beam.

The present invention is that when the crystalline silicon film obtainedby thermally crystallizing the amorphous silicon film formed on theglass substrate, patterned, machined and shaped crystalline siliconfilm, or those crystalline silicon film being added with an impurity issubjected to a laser annealing by scanning the linear laser beam, thewarp of the glass substrate caused by the thermally annealing step isannealed on the stage on which the glass substrate is mounted by a laserbeam in a forcedly flattened state.

In the present invention, the flattening of the glass substrate is tocorrect the glass substrate in such a manner that some external force isapplied to the glass substrate to reduce the warp of the substrate inthe state where the glass substrate is located on the stage.

The flattening of the glass substrate is conducted to the degree thatthe crystalline silicon film on the glass substrate is formed can beuniformly annealed using a linear laser beam.

A difference in level within the surface of the crystalline silicon filmon the glass substrate may be reduced so that the crystallinity of thecrystalline silicon film falls within a range which unifies into arequired level after the laser annealing.

In the present invention, with the glass substrate being mounted flatly,the linear laser beam is irradiated uniformly onto the crystallinesilicon film which is a surface to be irradiated without anydisplacement of the focal point regardless of the glass substrate per sebeing curved.

As a result, the crystalline silicon film having the uniformcrystallinity and mobility with the same quality within the substratesurface can be obtained.

According to the present invention, the crystalline silicon film on theglass substrate which has been warped after the thermallycrystallization has been conducted is corrected to the degree that thewarp of the glass substrate can be ignored, thereby irradiating a linearlaser beam thereon.

Therefore, the focal point of the linear laser beam on the surface to beirradiated can be prevented from shifting. As a result, even though thelaser annealing is conducted on the substrate by scanning the linearlaser beam, the uniform crystallization can be conducted, and thethreshold voltage of the TFT on which the film is formed can be unifiedwithin the substrate surface.

Because the degree of warp is more remarkable as the glass substrate isincreased in size, the effect of the present invention becomes moreremarkable as the glass substrate is increased in size.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1F show a producing process in accordance with an embodimentof the present invention;

FIG. 2 shows the distribution of the threshold voltage of a TFT using acrystalline silicon film formed in accordance with a conventional methodwithin a substrate surface;

FIG. 3 shows a state in which a silicon film formed on the glasssubstrate which has been warped is annealed by a laser beam;

FIG. 4 shows a TFT formed on the glass substrate;

FIG. 5 shows the distribution of the threshold voltage of a TFT using acrystalline silicon film formed in accordance with the embodiment withina substrate surface;

FIGS. 6A to 6C show the producing method of the embodiment;

FIGS. 7A to 7C show the producing method of the embodiment;

FIG. 8 shows the producing method of the embodiment;

FIG. 9 shows a substrate heating unit;

FIG. 10 shows the producing method of the embodiment;

FIG. 11 shows a laser irradiating method;

FIG. 12 is a conceptual diagram showing a laser annealing unit used inthe embodiment;

FIGS. 13A to 13C show an optical system;

FIGS. 14A and 14B show an optical system;

FIGS. 15A and 15B show the distribution of energy of a laser beam;

FIG. 16 show the distribution of the energy density of a laser beamprocessed into a linear shape in the widthwise direction of the laserbeam;

FIGS. 17A to 17D are structural diagrams showing another stage; and

FIGS. 18A to 18C show a laser irradiating process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(First Embodiment)

FIGS. 1A to 1F show a process of producing a thin film transistor (TFT)in accordance with a first embodiment.

An under silicon oxide film 102 having a thickness of 2000 Å is formedon a glass substrate (in this embodiment, using a Corning 7059 of 100 mmsquare), and an amorphous silicon film 103 having a thickness of 500 Åis formed sequentially on the under silicon oxide film 102 by plasmaCVD.

Then, in order to add nickel as a catalytic element that promotescrystallization, nickel acetate aqueous solution of 10 ppm is coated onthe surface of silicon, and nickel acetate layer not shown is formedthrough the spin coating technique.

It is more preferable that a surface active agent is added to the nickelacetate aqueous solution. Since the nickel acetate layer is very thin,although it is not limited to a film shape, it does not suffer from aproblem in the subsequent process (FIG. 1A).

Then, the glass substrate 101 is located on a stage having a surfaceflattened with high accuracy (the roughness and waviness of the surfaceis 5 μm or less), and then thermally annealed at 640° C. for four hours,to flatten glass substrate 101 and crystallize the amorphous siliconfilm. In this state, nickel serves as the core of crystal, to promotethe crystallization of the amorphous silicon film.

The strain point of the Corning 7059 substrate is 593° C., and thesoftening point is 844° C., and the annealing temperature of 640° C. isbetween the strain point and the softening point. Also, the annealingpoint of the Corning 7059 is 639° C.

In thermally crystallizing the substrate, that the processing can bemade at a low temperature for a short period of time, that is, at 640°C. for four hours is caused by the function of nickel. The details aredisclosed in Japanese Patent Unexamined Publication No. 6-244104.

The above publication discloses that the thermal annealing, for example,at 550° C. (below the strain point) for four hours is conducted so thatthe temperature in thermally annealing does not exceed the strain pointof the glass substrate. However, the temperature is determined so thatthe glass substrate is deformed as little as possible in thermallycrystallizing.

The present invention is to raise the substrate temperature up to atemperature at which the glass substrate is liable to be deformed asmuch as possible reversely so that the crystallization as well as theflattening of the substrate is conducted simultaneously.

It is preferable that the concentration of a catalytic element is 1×10¹⁵to 1×10¹⁹ atoms/cm³. When it is a high concentration equal to or morethan 1×10¹⁹ atoms/cm³, a metallic nature is exhibited on silicon so thatthe semiconductor characteristic disappears. The concentration of thecatalytic element in the silicon film in this embodiment is 1×10¹⁷ to5×10¹⁸ atoms/cm³ at the minimum in the film. Those values are theminimum values of the concentration of the catalytic elements in thesilicon film which has been analyzed and measured by the secondary ionmass spectrometry (SIMS).

In this way, the crystallization of the silicon film and the flatteningof the substrate are conducted, and after the completion of the process,the substrate is cooled up to the room temperature at 2° C./min.

In the case where the catalytic element that promotes crystallization isnot added to the silicon film, if the heating temperature is low, thenthere is a case in which only the flattening of the substrate isconducted in the above process, and no crystallization is conducted.However, that the uniform crystallization can be conducted in thesubsequent laser annealing process is identical with the case in whichthe catalytic elements are added to the silicon film.

In order to further enhance the crystallinity of the crystalline siliconfilm, an excimer laser beam which is a high-power pulse laser isirradiated onto the film.

In this embodiment, a KrF excimer laser (248 nm in wavelength and 30nsec in pulse width) is processed into a linear shape before being used.The size of a laser beam is 1×125 mm². The irradiation of the laser beamis conducted with the energy density of the laser beam being within 100mJ/cm² to 500 mJ/cm², for example, 370 mJ/cm².

The crystallinity is further enhanced by irradiating a laser beam ontothe film with an energy of about 220 mJ/cm² in advance before the aboveirradiation of the laser beam.

The laser irradiating method is as follows. The linear laser beam isirradiated onto the film while being shifted relatively with respect toan object to be irradiated. A direction in which the linear laser beamis shifted is substantially perpendicular to the line direction. In thissituation, paying an attention to one point of a surface to beirradiated, the laser beams of 2 to 20 shots are irradiated onto thepoint. Also, the substrate temperature at the time of irradiating thelaser beam is 200° C. (FIG. 1B).

A TFT is fabricated on the crystalline silicon film 104. The TFT isdisposed on the substrate in the form of a matrix. Specifically, TFTs of400×300 pieces are fabricated in a fabrication area of 40×50 mm². Thisprocess will be described below.

A silicon film is so etched as to form an island silicon region 105.Then, a silicon oxide film 106 having a thickness of 1200 Å is formed asa gate insulating film by plasma CVD. The raw material gas of the plasmaCVD as used is TEOS and oxygen. The substrate temperature when formingthe film on the substrate is 250 to 380° C., for example, 300° C. (FIG.1C).

Sequentially, an aluminum film (containing silicon of 0.1 to 2% therein)having a thickness of from 3000 to 8000 Å, for example, 600 Å isdeposited by sputtering. Then, the aluminum film is so etched as to forma gate electrode 107 (FIG. 1C).

Then, an impurity (boron) is implanted into a silicon region with a maskof the gate electrode 109 by ion doping. The doping gas as used isdiborane (B₂ H₆) which has been diluted with hydrogen into 1 to 10%, forexample, 5%.

An accelerating voltage is 60 to 90 kV, for example, 65 kV, the doseamount is 2×10¹⁵ to 5×10¹⁵ atoms/cm², for example, 3×10¹⁵ atoms/cm². Thesubstrate temperature at the time of ion doping is the room temperature.As a result, p-type impurity regions 108 (source) and 109 (drain) areformed (FIG. 1D).

Then, in order to activate doped boron, annealing is optically conductedagain using the KrF excimer laser. The energy density of the laser beamis 100 to 350 mJ/cm², for example, 250 mJ/cm². The crystallinity isfurther enhanced by irradiating a laser beam onto the film with anenergy of about 170 mJ/cm² in advance before the irradiation of thelaser beam.

The laser irradiating method is as follows. The linear laser beam isirradiated onto the film while being shifted relatively with respect toan object to be irradiated. A direction in which the linear laser beamis shifted is substantially perpendicular to the line direction. In thissituation, paying an attention to one point of a surface to beirradiated, the laser beams of 2 to 20 shots are irradiated onto thepoint. Also, the substrate temperature at the time of irradiating thelaser beam is 200° C. Thereafter, annealing is thermally conducted inthe nitrogen atmosphere at 450° C. for 2 hours (FIG. 1E).

A silicon oxide film 110 having a thickness of 6000 Å is formed as aninterlayer insulator by plasma CVD, in which contact holes are defined.Then, electrodes-wirings 111 and 112 of the source and the drain of theTFT are formed of a multi-layer film made of a metal material, forexample, titanium and aluminum. Finally, annealing is thermallyconducted at 200 to 350° C. under the hydrogen atmosphere of 1atmospheric pressure (FIG. 1F).

FIG. 5 shows the distribution of the threshold values of the TFT using acrystalline silicon film formed in accordance with this embodimentwithin the substrate surface.

The axis of abscissa in FIG. 5 corresponds to the respective locationsof the TFT shown in FIG. 4 (a portion surrounded by a dotted line inFIG. 4) as in the case of FIG. 2.

In FIG. 5, the TFT fabricated by this embodiment has a uniform thresholdvalue within the substrate surface. It is apparent that the TFT in FIG.5 has, within the substrate surface, a uniform threshold voltage morethan the conventional example shown in FIG. 2.

(Second Embodiment)

A second embodiment will be described with reference to FIGS. 1A to 1F.An under silicon oxide film 102 having a thickness of 2000 Å is formedon a glass substrate 101 (In this embodiment, there is used a Corning1737 of 400×500 mm square and 0.7 mm thickness. Another glass substratesuch as Corning 7059, OA2, NA45, etc., may be used.), and an amorphoussilicon film 103 having a thickness of 500 Å is formed sequentially onthe under silicon oxide film 102 by plasma CVD.

Then, nickel acetate aqueous solution of 10 ppm is coated on the surfaceof silicon, and a nickel acetate layer not shown is formed by spincoating.

It is more preferable that a surface active agent is added to the nickelacetate aqueous solution. Since the nickel acetate layer is very thin,although it is not necessarily to be in a film shape, it does not sufferfrom any problem in the subsequent process (FIG. 1A).

Then, the glass substrate 101 is thermally annealed at 550° C. for fourhours, to crystallize the amorphous silicon film 103. In this state,nickel serves as the core of crystal, to promote the crystallization ofthe amorphous silicon film. The strain point of the Corning 1737substrate is 667° C., and the annealing temperature of 550° C. is underthe strain point.

After the thermal crystallization, when the glass substrate is cooled,the silicon film contracts so that the substrate produces the concavewarp.

That the processing can be made at a low temperature (the strain pointof the Corning 1737 or less) for a short period of time, that is, at550° C. for four hours is caused by the function of nickel. The detailsare disclosed in Japanese Patent Unexamined Publication No. 6-244104.The publication discloses that the thermal annealing, for example, at550° C. (below the strain point) for four hours is conducted so that thetemperature in thermally annealing does not exceed the strain point ofthe glass substrate. However, the temperature is determined so that theglass substrate 101 is prevented from being remarkably deformed inthermally crystallizing.

In order to correct the warp of the glass substrate 101 after the abovethermally crystallizing process, the glass substrate 601 is put on abase 602 having a convex curved surface as shown in FIG. 6A, and thenappropriately heated (at 350 to 600° C. for several hours). The convexcurved surface has a curved surface which is substantially symmetricwith the warp of the glass substrate.

Then, in FIG. 6B, the glass substrate 601 is deformed along the shape ofthe base 602 by self-weight and heat. In this state, when the glasssubstrate is cooled, the silicon film 600 formed on the substratecontracts more markedly than the glass substrate 601, with the resultthat an extremely flat glass substrate 601 can be obtained.

In order to further enhance the crystallinity of the crystalline siliconfilm thus obtained, an excimer laser beam which is a high-power pulselaser is irradiated onto the film.

The outline of the laser annealing unit will be described below.

FIG. 12 shows a conceptual diagram of a laser annealing unit used inthis embodiment of the present invention. The laser annealing unit shownin FIG. 12 is of the multi-chamber system which is designed in such amanner that a substrate taken in from a loader/unloader chamber 11 andpositioned at an alignment chamber 12 is transferred to the respectivechambers through a transfer chamber 13 by a substrate transfer robot 14disposed in the transfer chamber 13 so that each substrate issequentially processed.

The substrate is firstly taken in the thermal chamber 15, and afterbeing subjected to the heat treatment such as the preliminary heating, alaser annealing is conducted in the laser annealing chamber. Then, afterthe substrate is conveyed to the cooling chamber 17 and cooled, it ismoved to the loader/unloader chamber 11, and moved outward.

The dispersion of the energy for each pulse of the laser annealing unitis 3σ within ±3%.

A pulse laser larger in dispersion than the above laser may be used, butthe focal depth is narrowed, the dispersion of 3σ exceeding ±10% is notapplicable to the present invention.

An oscillator as used is EX748 made by LUMNICS Corporation. The laserbeam as oscillated is a KrF excimer laser beam (248 nm in wavelength and25 ns in pulse width).

Another excimer laser as well as another type laser can be used. Itshould be noted that a laser beam of a pulse oscillator need to be used.

The laser annealing unit has a sealing property against the surroundingsso as to prevent the contamination due to an impurity. Also, it has anatmosphere control function when irradiating a laser beam. Also, thelaser annealing unit has a function for heating the substrate so that anobject to be irradiated when irradiating a laser beam can be kept to adesired temperature.

The oscillated laser beam is introduced into an optical system as shownin FIGS. 13A and 13B because the shape of the laser beam is deformed.

The laser beam immediately before being shone onto the optical system isof the rectangular of about 3×2 cm², however the laser beam is processedinto a slender beam (linear beam) of about 10 to 30 cm in length and0.01 to 0.3 cm in width, depending on the optical system.

Also, the distribution of the energy density of the linear laser beamthat has passed through the optical system in the cross direction istrapezoidal as shown in FIG. 15B. The energy of the laser beam that haspassed through the optical system is 800 mJ/shot at the maximum.

The reason why the laser beam is processed into such a slender beam isto improve the processability. When the linear beam is irradiated ontoan sample, if the length of the laser beam is longer than the width ofthe sample, then the sample is moved in one direction, therebyirradiating a laser beam onto the entire sample.

Even though the length of the beam is shorter than the width of thesample, troublesomeness in processing is saved in comparison with therectangular beam. However, in this case, there occurs the necessity ofmoving the beam vertically and horizontally relatively with respect tothe sample.

The stage (base) for the substrate (sample) on which a laser beam isirradiated is controlled by a computer and so designed as to be movedperpendicularly to the line direction of the linear laser beam. Also, itis so designed that the height of the substrate can fluctuate.

If the stage is provided with a function for moving in the linedirection of the laser beam, even though the beam width is shorter thanthe sample, a laser processing on the entire sample is enabled.

The optical path in the interior of the optical system that processes alaser beam into a linear laser beam will be described.

The laser beam incident to the optical system passes through acylindrical concave lens B, a cylindrical convex lens C (the lenses Band C are generally called "beam expander"), and fly eye lenses D andD2.

The laser beam passes through a cylindrical convex lens E as a firstcylindrical lens, and a cylindrical convex lens F as a secondcylindrical lens provided for improving the uniformity of the linearbeam in the line direction, and is then converged by the cylindricalconvex lens H via a mirror G before being irradiated onto the surface tobe irradiated.

A distance between the cylindrical lenses A and B is 230 mm, a distancebetween the fly eye lenses D and D2 is 230 mm, a distance between thefly eye lens D and the cylindrical lens E is 650 mm, and a distancebetween the cylindrical lens F and the surface to be irradiated is 650mm (the sum of the focal distances of the respective lenses). It isneedless to say that these distances can be changed in accordance withthe circumstances. The cylindrical lens H as used has a focal distanceof 120 mm.

The shape of the energy distribution of the focal points of the laserbeam is made trapezoidal by changing the lens H vertically(J-direction).

The surface to be irradiated is moved vertically (J-direction)relatively with respect to the lens H, thereby being capable ofdeforming the shape of the energy distribution of the laser beam on thesurface to be irradiated (focal point) with a range of from a nearlysquare shape to a nearly trapezoidal shape (refer to FIG. 13C). In orderto more sharpen those shapes, a slit may be inserted in the laseroptical path.

The optical system is not particularly limited if it is so designed asto deform the laser beam to the shape required by the present invention.

The laser beam is shaped into a linear form, and the area of a laserbeam on the surface to be irradiated is 125 mm×1 mm. The width of thelinear laser beam is half width of the maximum energy value of the laserbeam.

The energy profile (energy distribution) of the linear laser beam in thecross direction has an artificial trapezoidal distribution of L1=0.4 mmand L2, L3=0.25 mm as shown in FIG. 15B, which satisfies the inequalityof 0.5L1≦L2≦L1, 0.5L≦L3≦L1. In this case, the focal depth can be about±400 μm.

The degree of widening of the bottom of the trapezoidal distribution ischanged in accordance with a distance between the final lens of thelaser optical system and the surface to be irradiated. A distancebetween the final lens of the laser optical system and the surface to beirradiated is changed by the roughness of the object to be irradiatedduring the laser processing.

With any change of the distance between the final lens and the surfaceto be irradiated, the degree of widening of the bottom of thetrapezoidal distribution of the laser beam is changed. If a range of thechange is within a range of the above inequality, then the focal depthof about ±400 μm is obtained, and therefore when the roughness of thesurface to be irradiated is ±400 μm or less, the uniform laserprocessing is enabled.

On the contrary, the general laser beam having a square energydistribution is about ±200 μm or less in focal depth, and is adverselyaffected by the roughness of the surface to be irradiated and adifference in level, thereby being liable to make the crystallinitywithin the substrate surface nonuniform.

The sample is put on the stage (base) within the laser annealing chamber16 shown in FIG. 12, and a laser beam is irradiated while the stage ismoving at 2 mm/s. The laser irradiation conditions are that the energydensity of the laser beam is 100 to 500 mJ/cm², in this example, 300mJ/cm², and the number of pulses is 30 pulses/s. The energy densitymeans the density of the upper bottom portion (a portion having amaximum value) of the trapezoidal laser beam. The substrate temperaturewhen irradiating the laser beam is 200° C.

The irradiation of the laser beam is conducted under the aboveconditions. Paying an attention to a certain point of the sample, thelaser beams are irradiated at 15 steps. This is because, since it takes0.5 sec to allow one laser beam to pass, 15 pulses are irradiated on onelocation by irradiating the laser beam in the scanning manner. In thisexample, in the above 15 irradiations, the initial several irradiationshave the irradiated energy density gradually increased, and the finalseveral irradiations have the irradiated energy density graduallydecreased.

This state is schematically shown in FIG. 16. In the first half of 15steps, the laser energy gradually increases (see A in FIG. 16) whereas,in the latter half thereof, it gradually decreases (see B in FIG. 16).

With such an irradiation of the laser beam, the use of a single pulselaser beam can provide the same effect as the conventional two-stepirradiation system using a weak pulse laser beam for preliminary heatingand a strong pulse laser beam for crystallization.

Since the energy applied to a region to be irradiated is not rapidlychanged, a phase is not rapidly changed in the silicon film, and theroughness of the surface, the storage of the internal stress, etc., areprevented, thereby obtaining a uniform crystallinity.

Also, in this example, the atmospheric control is not particularlyconducted, and the irradiation of the laser beam is conducted in theatmosphere. It may be conducted under the vacuum or the atmosphere of aninactive gas such as argon or helium, hydrogen or nitrogen (FIG. 1B).

Then, a TFT is fabricated as a semiconductor device using thecrystalline silicon film thus fabricated in accordance with theproducing process as in the first embodiment. The TFT is arranged in theform of a matrix on the substrate. Specifically, TFTs of 400×300 piecesare fabricated in a producing area of 40×50 mm².

The distribution of the threshold values of the TFTs using thecrystalline silicon film formed in accordance with this embodimentwithin the substrate surface is uniform as shown in FIG. 5 as in thefirst embodiment.

(Third Embodiment)

Although in the second embodiment, the glass substrate 101 of 400×500 mmsquare is used, in a third embodiment, Corning 7059 of 100 mm square isused for a glass substrate. Hence, in flattening the glass substratewhich has been subjected to the crystallizing process, the shape of astage on which the glass substrate shown in FIG. 6A is mounted may be ofthe inverse U-shape type convex curved surface which is curved in onedirection.

The glass substrate is put on the stage having the inverse U-shape typeconvex curved surface, and an appropriate heat is applied to the glasssubstrate. Then, the glass substrate is deformed along the stage byself-weight and heat. In this situation, when the glass substrate iscooled, the silicon film formed on the substrate contracts more sharplythan the glass substrate, with the result that an extremely flat glasssubstrate can be obtained.

Thereafter, a TFT is fabricated in the same manner as that of the firstembodiment.

The distribution of the threshold voltage of the TFT is extremelyuniform within the substrate surface in comparison with the TFTmanufactured without flattening the glass substrate, as in the firstembodiment.

(Fourth Embodiment)

A fourth embodiment will be described with reference to FIGS. 1A to 1F.

An under silicon oxide film 102 having a thickness of 2000 Å is formedon a glass substrate 101 (In this embodiment, there is used a Corning1737 of 400×500 mm square and 0.7 mm thickness. It is needless to saythat another glass substrate such as Corning 7059, OA2, NA45, etc., maybe used.), and an amorphous silicon film 103 having a thickness of 500 Åis sequentially formed on the under silicon oxide film 102 by plasmaCVD.

Then, nickel acetate aqueous solution of 10 ppm is coated on the surfaceof silicon, and a nickel acetate layer is formed by spin coating. It ismore preferable that a surface active agent is added to the nickelacetate aqueous solution. Since the nickel acetate layer is very thin,although it is not necessarily to be in a film shape, it does not sufferfrom any problem in the subsequent process (FIG. 1A).

Then, the glass substrate is located on the convex stage (the risingportion in the center of a region on which the substrate is mounted ishigher in level than the edge of that region), and then thermallyannealed at 550° C. for four hours, to crystallize the amorphous siliconfilm.

In this situation, the glass substrate is deformed along the stage byself-weight and heat.

Also, in this state, nickel serves as the core of crystal, to therebypromote the crystallization of the silicon film. It should be noted thatthe strain point of the Corning 1737 substrate is 667° C., and theannealing temperature of 550° C. is below the strain point.

That the processing can be made at a low temperature (the strain pointof the Corning 1737 or less) for a short period of time, that is, at550° C. for four hours is caused by the function of nickel. The detailsare disclosed in Japanese Patent Unexamined Publication No. 6-244104.The publication discloses that the thermal annealing, for example, at550° C. (below the strain point) for four hours is conducted so that thetemperature in thermally annealing does not exceed the strain point ofthe glass substrate. However, the temperature is determined so that theglass substrate is prevented from being remarkably deformed in thermallycrystallizing.

It is preferable that the concentration of a catalytic element is 1×10¹⁵to 1×10¹⁹ atoms/cm³. When it is a high concentration equal to or morethan 1×10¹⁹ atoms/cm³, a metallic nature is exhibited on silicon,whereby the semiconductor characteristic disappears. The concentrationof the catalytic element in the silicon film in this embodiment is1×10¹⁷ to 5×10¹⁸ atoms/cm³ at the minimum in the film. It should benoted that those values are the minimum values of the concentration ofthe catalytic elements in the silicon film which has been analyzed andmeasured by the secondary ion mass spectrometry (SIMS).

After the thermal crystallization, when the glass substrate is cooled,the glass substrate is flattened because its coefficient of contractionis larger than that of the glass substrate.

In order to further enhance the crystallinity of the crystalline siliconfilm thus obtained, an excimer laser beam which is a high-power pulselaser is irradiated onto the film. A KrF excimer laser (248 nm inwavelength and 30 nsec in pulse width) is processed into a linear shapebefore being used. The size of a laser beam is 1×125 mm². Theirradiation of the laser beam is conducted with the energy density ofthe laser beam being within 100 mJ/cm² to 500 mJ/cm², for example, 370mJ/cm². The crystallinity is further enhanced by irradiating a laserbeam onto the film with an energy of about 220 mJ/cm² in advance beforethe above irradiation of the laser beam.

The laser irradiating method is as follows. The linear laser beam isirradiated onto the film while being shifted relatively with respect toan object to be irradiated. A direction in which the linear laser beamis shifted is substantially perpendicular to the linear laser. In thissituation, paying an attention to one point of a surface to beirradiated, the laser beams of 2 to 20 shots are irradiated onto thepoint. Also, the substrate temperature at the time of irradiating thelaser beam is 200° C. (FIG. 1B).

Thereafter, a TFT is fabricated in the same manner as that of the firstembodiment.

The distribution of the threshold voltage of the TFT thus obtained isextremely unified within the substrate surface in comparison with theTFT manufactured without flattening the glass substrate, as in the firstembodiment.

Also, although in this embodiment, the glass substrate 101 of 400 mm×500mm square is used, in the case of using Corning 7059 of 100 mm square asthe glass substrate 101 as in the third embodiment, the shape of thestage on which the glass substrate shown in FIG. 6A is mounted may bechanged to the inverse U-shape type convex curved surface which iscurved in one direction, in flattening the glass substrate which hasbeen crystallized.

(Fifth Embodiment)

A fifth embodiment will be described with FIGS. 1A to 1F.

An under silicon oxide film 102 having a thickness of 2000 Å is formedon a glass substrate 101 (In this embodiment, there is used a Corning1737 of 400×500 mm square and 0.7 mm thickness. Another glass substratesuch as Corning 7059, OA2, NA45, etc., may be used.), and an amorphoussilicon film 103 having a thickness of 500 Å is then formed on the undersilicon oxide film 102 by plasma CVD. Then, nickel acetate aqueoussolution of 10 ppm is coated on the surface of silicon, and a nickelacetate layer is formed by spin coating. It is more preferable that asurface active agent is added to the nickel acetate aqueous solution.Since the nickel acetate layer is very thin, although it is notnecessarily to be in a film shape, it does not suffer from any problemin the subsequent process (FIG. 1A).

Then, the glass substrate is thermally annealed at 550° C. for fourhours, to crystallize the amorphous silicon film. In this state, nickelserves as the core of crystal, to promote the crystallization of thesilicon film. The strain point of the Corning 1737 substrate is 667° C.,and the annealing temperature of 550° C. is below the strain point.

After the thermal crystallization, when the glass substrate is cooled,the silicon film contracts, to thereby warp the glass substrate in theform of a concave.

That the processing can be made at a low temperature (the strain pointof the Corning 1737 or less) for a short period of time, that is, at550° C. for four hours is caused by the function of nickel. The detailsare disclosed in Japanese Patent Unexamined Publication No. 6-244104.The publication discloses that the thermal annealing, for example, at550° C. (below the strain point) for four hours is conducted so that thetemperature in thermally annealing does not exceed the strain point ofthe glass substrate. However, the temperature is determined so that theglass substrate is prevented from being remarkably deformed in thermallycrystallizing.

It is preferable that the concentration of a catalytic element is 1×10¹⁵to 1×10¹⁹ atoms/cm³. When it is a high concentration equal to or morethan 1×10¹⁹ atoms/cm³, a metallic nature is exhibited on silicon,whereby the semiconductor characteristic disappears. The concentrationof the catalytic element in the silicon film in this embodiment is1×10¹⁷ to 5×10¹⁸ atoms/cm³ at the minimum in the film. It should benoted that those values are the minimum values of the concentration ofthe catalytic elements in the silicon film which has been analyzed andmeasured by SIMS.

In order to further enhance the crystallinity of the crystalline siliconfilm thus obtained, an excimer laser beam which is a high-power pulselaser is irradiated onto the film while heating the film. In thissituation, the glass substrate which has been warped in the form of aconcave is flattened simultaneously.

In this embodiment, a KrF excimer laser (248 nm in wavelength and 30nsec in pulse width) is used. The size of a laser beam is 30×20 mm². Theirradiation of the laser beam is conducted with the energy density ofthe laser beam being within 100 mJ/cm² to 500 mJ/cm², for example, 370mJ/cm². The crystallinity is further enhanced by irradiating a laserbeam onto the film with an energy of about 220 mJ/cm² in advance beforethe above irradiation of the laser beam.

The laser irradiating method is as follows:

The glass substrate, as shown in FIG. 8, is mounted on a convex stage,and the edges of the glass substrate are fixedly pushed by appropriatepushers 803 made of metal or the like, to thereby deform the substrateinto a convex shape.

The stage has a mechanism that allows heated helium gas to flow out andcirculate by a pump 904, under the substrate 901, as shown in FIG. 9, tothereby keep the substrate at a desired temperature.

In this situation, a laser processing is conducted on the substrate. Alaser beam is moved back and forth, right and left, and is irradiated onthe substrate so as to be overlapped with each other. Paying anattention to a certain point on the substrate, the number of times oflaser irradiation is 2 to 5.

It should be noted that, because the substrate to be irradiated iswarped in the form of a convex, the glass substrate is moved verticallywith respect to the laser beam so that the focal point of the laser beamis always put on the substrate. Since the thickness of the substrate,the shape of the convex surface, and so on are found in advance, theheight of the substrate is controlled on the basis of those data so thatannealing can be uniformly conducted on the convex substrate surface,keeping the focal point constant.

The height of the substrate may be fixed, and a lens is so adjust as tomove the focal point in such a manner that the focal point of the laserbeam is always set on the substrate.

Also, a distance to a surface to be irradiated is measured using adisplacement gauge or the like, on the basis of which the height of thesubstrate or the focal point may be automatically changed. Moreover, thesubstrate temperature at the time of irradiating a laser beam is 200° C.

Thereafter, the pushers are detached from the substrate, and thesubstrate is cooled, to thereby flatten the substrate with thecontraction of the silicon film (FIG. 1B).

In this manner, the silicon film uniform in crystallinity withinsubstrate surface and the flat substrate having the film can beobtained.

TFT is fabricated in the same manner as that of the first embodiment.

The distribution of the threshold voltage of the TFT is extremelyunified within the substrate surface in comparison with the TFTmanufactured without flattening the glass substrate, as in the firstembodiment.

(Sixth Embodiment)

A sixth embodiment will be described with reference to FIGS. 1A to 1F.

An under silicon oxide film 102 having a thickness of 2000 Å is formedon a glass substrate 101 (In this embodiment, there is used a Corning1737 of 400×500 mm square and 0.7 mm thickness. It is needless to saythat another glass substrate such as Corning 7059, OA2, NA45, etc., maybe used.), and an amorphous silicon film 103 having a thickness of 500 Åis sequentially formed on the under silicon oxide film 102 by plasmaCVD.

Then, nickel acetate aqueous solution of 10 ppm is coated on the surfaceof silicon, and a nickel acetate layer is formed by spin coating. It ismore preferable that a surface active agent is added to the nickelacetate aqueous solution. Since the nickel acetate layer is very thin,although it is not necessarily to be in a film shape, it does not sufferfrom any problem in the subsequent process (FIG. 1A).

Then, the glass substrate is thermally annealed at 550° C. for fourhours, to thereby crystallize the amorphous silicon film. In this state,nickel serves as the core of crystal, to thereby promote thecrystallization of the silicon film. It should be noted that the strainpoint of the Corning 1737 substrate is 667° C., and the annealingtemperature of 550° C. is below the strain point.

After the thermal crystallization, when the glass substrate is cooled,the silicon film contracts, to warp the glass substrate in the form of aconcave.

That the processing can be made at a low temperature (the strain pointof the Corning 1737 or less) for a short period of time, that is, at550° C. for four hours is caused by the function of nickel. The detailsare disclosed in Japanese Patent Unexamined Publication No. 6-244104.The publication discloses that the thermal annealing, for example, at550° C. (below the strain point) for four hours is conducted so that thetemperature in thermally annealing does not exceed the strain point ofthe glass substrate. However, the temperature is determined so that theglass substrate is prevented from being remarkably deformed in thermallycrystallizing.

It is preferable that the concentration of a catalytic element is 1×10¹⁵to 1×10¹⁹ atoms/cm³. When it is a high concentration equal to or morethan 1×10¹⁹ atoms/cm³, a metallic nature is exhibited on silicon,whereby the semiconductor characteristic disappears. The concentrationof the catalytic element in the silicon film in this embodiment is1×10¹⁷ to 5×10¹⁸ atoms/cm³ at the minimum in the film. It should benoted that those values are the minimum values of the concentration ofthe catalytic elements in the silicon film which has been analyzed andmeasured by SIMS.

In order to further enhance the crystallinity of the crystalline siliconfilm thus obtained, an excimer laser beam which is a high-power pulselaser is irradiated onto the film while heating the film. In thissituation, the glass substrate which has been warped in the form of aconcave is flattened simultaneously.

The laser irradiating method is as follows

The laser annealing unit shown in FIG. 12 is used as in the firstembodiment.

An oscillator as used is a model 3000-308 made by Lambda PhysicCorporation. The laser beam oscillated from the oscillator is an XeClexcimer laser beam (308 nm in wavelength and 26 ns in pulse width).

Another excimer laser as well as another type laser can be used. Itshould be noted that a laser beam of a pulse oscillator need to be used.The oscillated laser beam is introduced into an optical system as shownin FIGS. 14A and 14B in order to deform the beam shape.

The laser beam immediately before being shone onto the optical system isrectangular to the degree of 3×2 cm², but it is processed by the opticalsystem into a slender beam (linear beam) of about 10 to 30 cm in lengthand 0.01 to 0.3 cm in width.

The distribution of the energy density of the linear laser beam that haspassed through the optical system in the cross direction is trapezoidalas shown in FIG. 15B. The energy of the laser beam that has passedthrough the optical system is 1000 mJ/shot at the maximum.

The reason why the laser beam is processed into such a slender beam isto improve the processability. When the linear beam is irradiated ontoan sample, if the length of the laser beam is longer than the width ofthe sample, then the sample is moved in one direction, therebyirradiating a laser beam onto the entire sample.

Even though the length of the beam is shorter than the width of thesample, troublesomeness in processing is saved in comparison with therectangular beam. However, in this case, there occurs the necessity ofmoving the beam vertically and horizontally relatively with respect tothe sample.

The stage (base) for the substrate (sample) on which a laser beam isirradiated is controlled by a computer and so designed as to be movedperpendicularly to the line direction of the linear laser beam. Also, itis so designed that the height of the substrate can fluctuate.

If the stage is provided with a function for moving in the linedirection of the laser beam, even though the beam width is shorter thanthe sample, a laser processing on the entire sample is enabled.

The optical path in the interior of the optical system that processes alaser beam into a linear laser beam will be described.

The laser beam oscillated from a laser light source a and incident tothe optical system first passes through fly eye lenses b and c.

The laser beam passes through a cylindrical convex lens d as a firstcylindrical lens, and a cylindrical convex lens e as a secondcylindrical lens provided for improving the uniformity of the linearbeam in the line direction, and is then converged by the cylindricalconvex lens g via a mirror f before being irradiated onto the sample.

A distance of from the laser light source to the mirror g is 2000 mm,and a distance of from the mirror f to the surface to be irradiated is440 mm. The cylindrical lens g as used has a focal distance of 100 mm.

The shape of the energy distribution of the laser beam at the focalpoint is made trapezoidal by changing the lens g vertically(i-direction).

The surface to be irradiated is moved vertically (i-direction)relatively with respect to the lens g, thereby deforming the shape ofthe energy distribution of the laser beam on the surface to beirradiated (focal point) with a range of from a nearly square shape to anearly trapezoidal shape.

The optical system is not particularly limited if it is so designed asto deform the laser beam to the shape required by the present invention.

The optical system is not limited to that shown in FIGS. 14A and 14B,but it may be provided with lenses B and C as shown in FIGS. 13A and13B.

The laser beam is shaped into a linear form, and the area of a laserbeam on the surface to be irradiated is 150 mm×0.4 mm. (The width of thelinear laser beam is half of the maximum energy value of the laserbeam.)

The energy profile (energy distribution) of the linear laser beam in thecross direction has an artificial trapezoidal distribution of L1=0.1 mmand L2, L3=0.08 mm as shown in FIG. 15B, which satisfies the inequalityof 0.5L1≦L2≦L1, 0.5L≦L3≦L1. In this case, the focal depth can be about±400 μm.

The degree of widening of the bottom of the trapezoidal distribution ischanged in accordance with a distance between the final lens of thelaser optical system and the surface to be irradiated. A distancebetween the final lens of the laser optical system and the surface to beirradiated is changed by the roughness of the object to be irradiatedduring the laser processing.

With any change of the distance between the final lens and the surfaceto be irradiated, the degree of widening of the bottom of thetrapezoidal distribution of the laser beam is changed. If a range of thechange is within a range of the above inequality, then the focal depthof about ±400 μm is obtained, and therefore when the roughness of thesurface to be irradiated is ±400 μm or less, the uniform laserprocessing is enabled.

The general laser beam having a square energy distribution is about ±200μm or less in focal depth.

The glass substrate, as shown in FIG. 10, is mounted on a U-shapedconvex stage, and the edges of the glass substrate are fixedly pushed byappropriate pushers made of metal or the like, to curve the substrateinto a U-shape.

The stage has a mechanism that allows heated helium gas to flow out andcirculate, under the substrate, as shown in FIG. 9, to thereby keep thesubstrate at a desired temperature. The laser processing is conductedwhile the linear laser beam is being shifted relatively with respect tothe object to be irradiated. A direction in which the linear laser beamis shifted is substantially perpendicular to the linear laser beam, anda straight line contained in the U-shaped curved surface of thesubstrate to be irradiated is substantially in parallel with the linearlaser beam.

Because the substrate to be irradiated is warped in the form of aU-shape, as shown in FIG. 11, the glass substrate is moved verticallywith respect to the laser beam so that the focal point of the laser beamis always put on the substrate, during the irradiation of a laser beam.

Since the thickness of the substrate, the shape of the convex surface,and so on are found in advance, the height of the substrate iscontrolled on the basis of those data so that annealing can be uniformlyconducted on the U-shaped surface of the substrate, keeping the focalpoint constant.

The height of the substrate may be fixed, and a lens is so adjust as tomove the focal point in such a manner that the focal point of the laserbeam is always set on the substrate.

Also, a distance to a surface to be irradiated is measured using adisplacement gauge or the like, on the basis of which the height of thesubstrate or the focal point may be automatically changed.

The substrate temperature at the time of irradiating a laser beam is200° C.

Since the energy distribution of a laser beam to be irradiated istrapezoidal, and the focal depth is about ±400 μm, if a difference inlevel between the central portion and the edge portion of the U-shapedconvex stage is about ±400 μm or less, the laser annealing can beuniformly conducted within the substrate surface even though the stageand the focal point are not varied at all.

The laser annealing can be extremely uniformly conducted if the stage orthe focal point is varied in accordance with a difference in level ofthe surface to be irradiated, using a laser beam having such a focaldepth.

The glass substrate on the stage is moved perpendicular to the linewidth direction at 2.5 mm/s.

The laser irradiation conditions are that the energy density of thelaser beam is 100 to 500 mJ/cm², in this example, 400 mJ/cm², and thenumber of pulses is 200 pulses/s. It should be noted that the energydensity means the density of the upper bottom portion (a portion havinga maximum value) of the trapezoidal laser beam.

The irradiation of the laser beam is conducted under the aboveconditions. Paying an attention to a certain point of the sample, thelaser beams are irradiated at 32 steps. This is because, since it takes0.4 sec to allow one laser beam to pass, 32 pulses are irradiated on onelocation by irradiating one laser beam in the scanning manner. In thisexample, in the above 32 irradiations, the initial several irradiationshave the irradiated energy density gradually increased, and the finalseveral irradiations have the irradiated energy density graduallydecreased.

This state is schematically shown in FIG. 16. In the first half of 32steps, the laser energy gradually increases (see A in FIG. 16) whereas,in the latter half thereof, it gradually decreases (see B in FIG. 16).

Also, in this example, the atmospheric control is not particularlyconducted, and the irradiation of the laser beam is conducted in theatmosphere. It may be conducted under the vacuum or the atmosphere of aninactive gas such as argon or helium, hydrogen or nitrogen.

Thereafter, the pushers are detached from the substrate and then cooledso that the substrate is flattened with the contraction of the siliconfilm (FIG. 1B).

The silicon film uniform in crystallinity within the substrate surfaceand the flat substrate having the film can be obtained. Thereafter, aTFT is fabricated in the same manner as in the first embodiment.

The distribution of the threshold voltage of the TFT thus obtained isextremely unified within the substrate surface in comparison with theTFT manufactured without flattening the glass substrate, as in the firstembodiment.

(Seventh Embodiment)

A seventh embodiment will be described with reference to FIGS. 1A to 1F.

An under silicon oxide film 102 having a thickness of 2000 Å is formedon a glass substrate 101 (In this embodiment, there is used a Corning1737 of 400×500 mm square and 0.7 mm thickness. It is needless to saythat another glass substrate such as Corning 7059, OA2, NA45, etc., maybe used.), and an amorphous silicon film 103 having a thickness of 500 Åis sequentially formed on the under silicon oxide film 102 by plasmaCVD.

In order to crystallize the amorphous silicon film, an excimer laserbeam which is a high-power pulse laser is irradiated onto the film whileheating the film.

A KrF excimer laser (248 nm in wavelength and 30 nsec in pulse width) isused. The size of a laser beam is 30×20 mm². The laser beam isirradiated with the energy density of the laser beam being within 100mJ/cm² to 500 mJ/cm², for example, 370 mJ/cm². The crystallinity isfurther enhanced by irradiating a laser beam onto the film with anenergy of about 220 mJ/cm² in advance before the irradiation of thelaser beam.

In this state, in order to prevent the substrate from being warped bythe contraction of the silicon film after the silicon film iscrystallized and cooled, the laser irradiating method is conducted asfollows:

The glass substrate, as shown in FIG. 8, is mounted on a convex stage,and the edges of the glass substrate are fixedly pushed by appropriatepushers made of metal or the like, to thereby deform the substrate intoa convex shape.

The stage has a mechanism that allows heated helium gas to flow out andcirculate under the substrate, as shown in FIG. 9, to thereby keep thesubstrate at a desired temperature.

In this situation, a laser processing is conducted on the substrate. Alaser beam is moved back and forth, right and left, and is irradiated onthe substrate so as to be overlapped with each other.

Paying an attention to a certain point on the substrate, the number oftimes of laser irradiation is 2 to 5.

Because the substrate to be irradiated is warped in the form of aconvex, the glass substrate is moved vertically with respect to thelaser beam so that the focal point of the laser beam is always put onthe substrate. Since the thickness of the substrate, the shape of theconvex surface, and so on are found in advance, the height of thesubstrate is controlled on the basis of those data so that annealing canbe uniformly conducted on the convex substrate surface, keeping thefocal point constant.

The height of the substrate is fixed, and a lens is so adjust as to movethe focal point in such a manner that the focal point of the laser beamis always set on the substrate.

Also, a distance to a surface to be irradiated is measured using adisplacement gauge or the like, on the basis of which the height of thesubstrate or the focal point may be automatically changed.

The substrate temperature at the time of irradiating a laser beam is200° C.

Thereafter, the pushers are detached from the substrate, and thesubstrate is cooled, to thereby flatten the substrate with thecontraction of the silicon film (FIG. 1B).

In this manner, the silicon film uniform in crystallinity withinsubstrate surface and the flat substrate having the film can beobtained. Thereafter, a TFT is fabricated in the same manner as in thefirst embodiment.

The distribution of the threshold voltage of the TFT thus obtained isextremely unified within the substrate surface in comparison with theTFT manufactured without flattening the glass substrate, as in the firstembodiment.

(Eighth Embodiment)

An eighth embodiment will be described with reference to FIGS. 1A to 1F.

An under silicon oxide film 102 having a thickness of 2000 Å is formedon a glass substrate 101 (In this embodiment, there is used a Corning1737 of 400×500 mm square and 0.7 mm thickness. It is needless to saythat another glass substrate such as Corning 7059, OA2, NA45, etc., maybe used.), and an amorphous silicon film 103 having a thickness of 500 Åis sequentially formed on the under silicon oxide film 102 by plasmaCVD.

In order to crystallize the amorphous silicon film, an excimer laserbeam which is a high-power pulse laser is irradiated onto the film whileheating the film. In this process, the glass substrate which has beenwarped in the form of a concave is flattened simultaneously.

In this embodiment, crystallization is conducted using a laser annealingunit having the optical system shown in FIGS. 14A and 14B, as in thefourth embodiment. Various conditions for the laser annealing are thesame as those in the fourth embodiment.

The glass substrate is mounted on a U-shaped convex stage as shown inFIG. 10, which is disposed in a laser annealing chamber of the laserannealing unit shown in FIG. 12, and the edges of the glass substrateare fixedly pushed by pushers made of metal or the like, to curve thesubstrate into a U-shape.

The stage has a mechanism that allows heated helium gas to flow out andcirculate under the substrate, as shown in FIG. 9, to maintain thesubstrate at a desired temperature.

The laser processing is conducted while the linear laser beam is beingshifted relatively with respect to the object to be irradiated. Adirection in which the linear laser beam is shifted is substantiallyperpendicular to the linear laser beam, and a straight line contained inthe U-shaped curved surface of the substrate to be irradiated issubstantially in parallel with the linear laser beam.

Because the substrate to be irradiated is warped in the form of a convexU-shape, as shown in FIG. 11, the glass substrate is moved verticallywith respect to the laser beam so that the focal point of the laser beamis always put on the substrate, during the irradiation of a laser beam.

Since the thickness of the substrate, the shape of the convex surface,and so on are found in advance, the height of the substrate iscontrolled on the basis of those data so that annealing can be uniformlyconducted on the U-shaped surface of the substrate, keeping the focalpoint constant.

The height of the substrate may be fixed, and a lens is so adjust as tomove the focal point in such a manner that the focal point of the laserbeam is always set on the substrate.

Also, a distance to a surface to be irradiated is measured using adisplacement gauge or the like, on the basis of which the height of thesubstrate or the focal point may be automatically changed.

Since the energy distribution of a laser beam to be irradiated istrapezoidal, and the focal depth is about ±400 μm, if a difference inlevel between the central portion and the edge portion of the U-shapedconvex stage is about ±400 μm or less, the laser annealing can beuniformly conducted within the substrate surface even though the stageand the focal point are not varied at all.

That the laser annealing can be extremely uniformly conducted if thestage or the focal point is varied in accordance with a difference inlevel of the surface to be irradiated, using a laser beam having such afocal depth.

Moreover, the substrate temperature at the time of irradiating a laserbeam is 200° C.

Thereafter, the pushers are detached from the substrate, and thesubstrate is cooled, to flatten the substrate with the contraction ofthe silicon film (FIG. 1B).

In this manner, the silicon film uniform in crystallinity withinsubstrate surface and the flat substrate having the film can beobtained. Thereafter, a TFT is fabricated in the same manner as in thefirst embodiment.

The distribution of the threshold voltage of the TFT is extremelyunified within the substrate surface in comparison with the TFT producedwithout flattening the glass substrate, as in the first embodiment.

(Ninth Embodiment)

A process of producing a TFT in accordance with a ninth embodiment willbe described with reference to FIGS. 1A to 1F.

An under silicon oxide film 102 having a thickness of 2000 Å is formedon a glass substrate 101 (In this embodiment, there is used a Corning7059 of 100 mm square. Another glass substrate such as Corning 1737,OA2, NA45, etc., may be used.), and an amorphous silicon film 103 havinga thickness of 500 Å is then formed on the under silicon oxide film 102by plasma CVD.

Then, nickel acetate aqueous solution of 10 ppm is coated on the surfaceof silicon, and a nickel acetate layer is formed by spin coating. It ismore preferable that a surface active agent is added to the nickelacetate aqueous solution. Since the nickel acetate layer is very thin,although it is not necessarily to be in a film shape, it does not sufferfrom any problem in the subsequent process (FIG. 1A).

Then, the glass substrate is thermally annealed at 550° C. for fourhours, to crystallize the amorphous silicon film. In this state, nickelserves as the core of crystal, to promote the crystallization of thesilicon film.

That the processing can be made at a low temperature (the strain pointof the Corning 7059 or less) for a short period of time, that is, at550° C. for four hours is caused by the function of nickel. The detailsare disclosed in Japanese Patent Unexamined Publication No. 6-244104.

It is preferable that the concentration of a catalytic element is 1×10¹⁵to 1×10¹⁹ atoms/cm³. When it is a high concentration equal to or morethan 1×10¹⁹ atoms/cm³, a metallic nature is exhibited on silicon,whereby the semiconductor characteristic disappears. The concentrationof the catalytic element in the silicon film in this embodiment is1×10¹⁷ to 5×10¹⁸ atoms/cm³ at the minimum in the film. It should benoted that those values are the minimum values of the concentration ofthe catalytic elements in the silicon film which has been analyzed andmeasured by SIMS.

In this way, a crystalline silicon film is obtained.

In this situation, the glass substrate is warped on the surface on whichthe crystalline silicon film is formed, so as to be concave. Adifference in level between the central portion and the peripheralportion of the substrate is about 50 μm. The degree of warp is differentdepending on the size, the thickness and the kind of the glasssubstrate.

In order to further enhance the crystallinity of the crystalline siliconfilm, an excimer laser beam which is a high-power pulse laser isirradiated onto the film.

In this embodiment, there is used the laser annealing unit describedwith reference to the second embodiment shown in FIGS. 12, 13A and 13B.

FIG. 17 shows an example of the structure of the stage.

As an example of means for fixedly mounting the flattened glasssubstrate on the stage, there is, for example, provided a plurality ofsuction inlets 1202 on the upper surface of a stage 1201. The suctioninlets 1202 are holes, and a flat surface is formed at portions where nosuction inlet 1202 exists.

FIG. 17B shows that a groove 1212 is defined in the upper surface of thestage 1211. The groove 1212 communicates with a suction inlet 1213 inthe center of the stage, and a flat surface is formed at portions whereno groove 1212 exists.

FIG. 17C shows that a plurality of protrusions 1222 are provided on theupper surface of a stage 1221, and a flat surface is formed by the uppersurface of those protrusions 1222 and the peripheral portion of thestage. Also, a suction inlet 1223 is provided for making vacuous.

FIGS. 17A to 17D show that the glass substrate is vacuum-sucked onto theflat surface by making vacuous through the suction inlet 1223. In thismanner, the lower surface of the glass substrate is in close contactwith the flat surface of the stage. Then, in this state, the laserannealing is conducted.

Because the flat surface on the upper surface of the stage is flatexcept for the portion taking part in the suction, the glass substratein contact with the above flat upper surface is flattened in accordancewith the flat surface of the stage.

In this vacuum-suction method, the location and the detachment of theglass substrate are performed extremely readily and for a short time.Also, because no obstruction that prevents the irradiation of a laserbeam exists on the glass substrate surface, the laser beam is uniformlyirradiated onto the entire surface of the glass substrate.

The flat surface of the stage is preferably as flat as possible.However, it is sufficient that the crystalline silicon film on the glasssubstrate mounted on the flat surface can be annealed using a linearlaser beam so that the film has the uniformity of a required level.

For example, the flat surface is so formed that a difference in level onthe surface to be irradiated, of the glass substrate is at least thefocal depth of the laser beam or less.

The method of bringing the glass substrate in close contact with thestage is not limited to the above suction method, and any kind of methodmay be used if the glass substrate can be flattened and the laserannealing can be conducted.

As another method, for example, in FIG. 17D, the peripheral portion orthe edge portions on the upper surface of the glass substrate 1101 aremechanically pushed and pressed (pressurized) against the flat surfaceof the stage 1231 by pushers 1232, and in this situation, the laserannealing may be conducted.

In that case, because the glass substrate can be flattened by a strongerforce than the vacuum suction, the glass substrate which is stronglywarped to the degree that it cannot be sufficiently flattened by vacuumsuction can be readily flattened.

The method shown in FIG. 17D and the above suction method may be usedtogether.

The material of the stage is preferably quartz, metal, ceramics or thelike because they are high in heat resistance and keep the flatnesshigh. In this example, there is used the stage having the structureshown in FIG. 17A.

The suction inlets 1202 shown in FIG. 17A are about 1 mm in diameter andprovided at the intervals of 10 mm.

The glass substrate 1101 is placed on the flat surface of the stage 1201in such a manner that a surface of the glass substrate 1101 on which acrystalline silicon film 1103 has been formed is directed upwardly, andvacuum is made from the suction inlets 1202 so that the glass substrate1101 is brought in close contact with the stage.

The glass substrate 1101 is also flattened to the same degree as adifference in level within the flat surface of the stage 1201 inaccordance with the flat surface of the stage 1201.

In addition to the structure of FIG. 17A, not only the glass substrate1101 is merely placed on the stage 1201, but also, after the former isplaced on the latter, vacuum is made in a state where a press is appliedto the upper surface of the substrate, in particular, the upper surfaceof the peripheral portion, and vacuum is then made so that the glasssubstrate is brought in close contact with the stage. For example,pushers 1232 shown in FIG. 17D is provided so that the upper surface ofthe peripheral portion of the glass substrate 1101 is pushed, and vacuumis made to bring the glass substrate in close contact with the stage.Then, the pushers are detached from the glass substrate, and thereafterthe laser annealing is conducted.

FIGS. 18A to 18C show a process of irradiating a laser beam inaccordance with this embodiment.

The amorphous silicon film formed on the glass substrate 2101 isthermally crystallized to obtain a crystalline silicon film 2103, andafter cooling, the glass substrate 2101 is warped. As shown in FIG. 18A,the glass substrate 2101 is located on the stage 2201.

In FIG. 18B, the warped glass substrate 2101 is forcedly corrected byflattening and mounting the glass substrate formed on the stage 2201, inthis example, by the suction inlet 2202. The glass substrate mounted onthe stage 2201 is flattened to the degree of about 5 μm in level.

In FIG. 18C, a linear laser beam is irradiated onto the crystallinesilicon film 2103 on the glass substrate which has been flattened in thescanning manner.

In this way, with the glass substrate 2101 being flatly mounted, thelinear laser beam is uniformly irradiated onto the crystalline siliconfilm which is a surface to be irradiated without any shift of the focalpoints regardless of the glass substrate per se being warped.

The irradiation of the laser beam is conducted with the energy densityof the laser beam being within 100 mJ/cm² to 500 mJ/cm², for example,370 mJ/cm². The crystallinity is further enhanced by irradiating a laserbeam onto the film with an energy of about 220 mJ/cm² in advance beforethe above irradiation of the laser beam, as the two-step irradiation.

The irradiation of the laser beam is conducted while the linear laserbeam is being shifted relatively with respect to an object to beirradiated, that is, a crystalline silicon film. A direction in whichthe linear laser beam is shifted is substantially perpendicular to thelinear laser beam (FIG. 14B, h-direction). In this situation, paying anattention to a certain point on the substrate, the laser beam of 2 to 40shots, for example, 32 shots is irradiated on the substrate. Also, thesubstrate temperature at the time of irradiating the laser beam is 200°C. (FIG. 1B).

In this way, a crystalline silicon film is fabricated. The crystallinesilicon film thus fabricated becomes sufficiently uniform because thedispersion of the mobility within the substrate surface is about ±10%.

In the crystalline silicon film which has been annealed by a laser beamnot through the flattening process shown in this embodiment, thedispersion of mobility within the substrate surface is about ±15 to 40%.Thus, the sufficient uniformity cannot be obtained.

On the basis of the crystalline silicon film thus fabricated, the TFTsof 400×300 pieces within the manufacture area of 40×50 mm² is fabricatedin accordance with the producing process as described in the firstembodiment.

The threshold voltage of the TFT is extremely unified within thesubstrate surface in comparison with the TFT manufactured withoutflattening the glass substrate, as shown in FIG. 5.

(Tenth Embodiment)

In the second embodiment, there is shown an example in which thearrangement of the optical system and the structure of the stage areused, which are different from those in the first embodiment.

As in the first embodiment, referring to FIGS. 1A to IF, an undersilicon oxide film 102 having a thickness of 2000 Å is formed on a glasssubstrate 101 (In this embodiment, there is used a Corning 1737 of300×300 mm square and 0.7 mm thickness. It is needless to say thatanother glass substrate such as Corning 7059, OA2, NA45, etc., may beused.), and an amorphous silicon film 103 having a thickness of 500 Å issequentially formed on the under silicon oxide film 102 by plasma CVD.

Then, nickel acetate aqueous solution of 10 ppm is coated on the surfaceof silicon, and a nickel acetate layer is formed by spin coating. It ismore preferable that a surface active agent is added to the nickelacetate aqueous solution. Since the nickel acetate layer is very thin,although it is not necessarily to be in a film shape, it does not sufferfrom any problem in the subsequent process (FIG. 1A).

Then, the glass substrate is thermally annealed at 550° C. for fourhours, to thereby crystallize the silicon film 103. In this state,nickel serves as the core of crystal, to promote the crystallization ofthe amorphous silicon film. The strain point of the Corning 1737substrate is 667° C., and the annealing temperature of 550° C. is belowthe strain point.

After the thermal crystallization, when the glass substrate is cooled,the silicon film contracts so that the substrate produces the concavewarp.

That the processing can be made at a low temperature (the strain pointof the Corning 1737 or less) for a short period of time, that is, at550° C. for four hours is caused by the function of nickel. The detailsare disclosed in Japanese Patent Unexamined Publication No. 6-244104.The publication discloses that the thermal annealing, for example, at550° C. (below the strain point) for four hours is conducted so that thetemperature in thermally annealing does not exceed the strain point ofthe glass substrate. However, the temperature is determined so that theglass substrate 101 is prevented from being remarkably deformed inthermally crystallizing.

In this state, the glass substrate is curved along the surface on whichthe crystalline silicon film is formed, into a concave portion.

In this example, a difference in level between the central portion andthe peripheral portion of the glass substrate is about 200 μm. Thedegree of the warp is different depending upon the size, the thicknessand the kind of the glass substrate.

In order to further enhance the crystallinity of the crystalline siliconfilm, an excimer laser beam which is a high-power pulse laser isirradiated onto the film.

The laser annealing unit has the structure shown in FIG. 6 as in thefirst embodiment.

An oscillator as used is an EX748 made by LUMNICS Corporation. The laserbeam oscillated from the oscillator is a KrF excimer laser beam (248 nmin wavelength and 25 ns in pulse width).

Another excimer laser as well as another type laser can be used. Itshould be noted that a laser beam of a pulse oscillator need to be used.

The oscillated laser beam is introduced into an optical system as shownin FIGS. 14A and 14B in order to deform the beam shape.

The laser beam immediately before being shone onto the optical system isrectangular to the degree of 3×2 cm², but it is processed by the opticalsystem into a slender beam (linear beam) of about 10 to 30 cm in lengthand 0.01 to 0.3 cm in width. The energy of the laser beam that haspassed through the optical system is 800 mJ/shot at the maximum.

The reason why the laser beam is processed into such a slender beam isto improve the processability. When the linear beam is irradiated ontoan sample, if the length of the laser beam is longer than the width ofthe sample, then the sample is moved in one direction, therebyirradiating a laser beam onto the entire sample.

Even though the length of the beam is shorter than the width of thesample, troublesomeness in processing is saved in comparison with therectangular beam. However, in this case, there occurs the necessity ofmoving the beam back and forth, right and left relatively with respectto the sample.

The stage (base) for the substrate (sample) on which a laser beam isirradiated is controlled by a computer and so designed as to be movedperpendicularly (FIG. 8, I-direction) to the line direction of thelinear laser beam.

If the stage is provided with a function for moving in the linedirection of the laser beam, even though the beam width is shorter thanthe sample, a laser processing on the entire sample is enabled.

The optical system that processes a laser beam into a linear laser beammay be the same as that in other embodiments.

The optical system is not limited if it can deform a laser beam into thebeam shape required by the present invention.

The laser beam is shaped into a linear form, and the area of a laserbeam on the surface to be irradiated is 300 mm×1 mm. The width of thelinear laser beam is half of the maximum energy value of the laser beam.

The glass substrate warped in the form of a concave through thethermally crystallizing process is forcedly flattened and fixed by thestage (base) of the laser annealing unit.

In this example, the stage having the structure shown in FIG. 17D isused. In FIG. 17D, the pusher 1232 in this example is made of ceramic,but it may be made of metal, quartz or the like. It is desirable that itmay be made of a material high in heat resistance and hard in thermalexpansion.

The pusher 1232, when the glass substrate 1101 is transferred andmounted on the stage 1231, is automatically pressed on the upperperipheral portion of the glass substrate 1101 in such a manner that theglass substrate 1101 is fixedly brought in close contact with the stage.

The glass substrate 1101 is flattened in accordance with the flatsurface of the stage 1231 and fixed thereto. The flattened glasssubstrate is about 10 μm in a difference in level within the surface.

In this manner, a laser beam is irradiated onto the glass substratelocated on the stage (base).

The irradiation of the laser beam is conducted while the linear laserbeam is being shifted relatively with respect to an object to beirradiated, that is, a crystalline silicon film. A direction in whichthe linear laser beam is shifted is substantially perpendicular to thelinear laser beam (FIG. 13B, I-direction). In this situation, paying anattention to a certain point on the substrate, the laser beam of 2 to 20shots, for example, 15 shots is irradiated on the substrate.

The irradiation of the laser beam is conducted with the energy densityof the laser beam being within 100 mJ/cm² to 500 mJ/cm², for example,370 mJ/cm². The crystallinity is further enhanced by irradiating a laserbeam onto the film with an energy of about 220 mJ/cm² in advance beforethe above irradiation of the laser beam, as the two-step irradiation.

Also, the substrate temperature at the time of irradiating the laserbeam is 200° C. (FIG. 1B).

Moreover, in this example, the atmospheric control is not particularlyconducted, and the irradiation of the laser beam is conducted in theatmosphere. It may be conducted under the vacuum or the atmosphere of aninactive gas such as argon or helium, hydrogen or nitrogen.

In this way, the crystalline silicon film having the uniformcrystallinity within the substrate surface can be obtained.

Thereafter, a TFT is fabricated using the crystalline silicon film as inthe first embodiment.

The threshold voltage of the TFT is extremely unified within thesubstrate surface in comparison with the TFT manufactured withoutflattening the glass substrate.

In the present invention, the substrate on which the film has beenformed can be restrained from being warped after being heated andcooled, whereby the substrate can be flattened.

In the present invention, the glass substrate on which the crystallinesilicon film is formed can be flattened, and the crystalline siliconfilm having the uniform and high crystallinity within the substratesurface can be obtained even after the laser irradiation process.

The crystalline silicon TFT uniform in the threshold value voltagewithin the substrate surface can be fabricated.

The present invention is effective particularly in the case where thearea of the glass substrate is large in producing a large number of TFTson the glass substrate.

In forming a liquid crystal display using the glass substrate, the cellpair can be made readily and surely since the substrate is flat.

As described above, the present invention is useful from the industrialviewpoint.

What is claimed is:
 1. A method of manufacturing a semiconductor devicecomprising the steps of:forming a semiconductor film comprisingamorphous silicon film over a glass substrate or over a silicon oxidefilm formed on a glass substrate; heating said glass substrate and saidamorphous silicon film on a convex curved substrate at a temperature ina range between a strain point and a softening point of said glasssubstrate, thereby forming said glass substrate into a convex curvedshape; flattening the glass substrate by cooling the glass substrate ina way that causes it to flatten; and conducting a laser annealingprocess on the silicon film.
 2. In a method according to claim 1, saidsemiconductor film is crystallized in said heating step.
 3. A method ofmanufacturing a semiconductor device comprising the steps of:forming asemiconductor film comprising amorphous silicon film over a glasssubstrate or over a silicon oxide film formed on said glass substrate;heating said glass substrate and said amorphous silicon film on a convexcurved surface at a temperature in a range between a strain point and asoftening point of said glass substrate, thereby forming said glasssubstrate into a convex curved shape; flattening the glass substrate bycooling said substrate in a way that causes it to flatten; conducting alaser annealing process of the silicon film; and forming a plurality ofthin film transistors each having an active layer using the silicon filmafter the laser annealing process.
 4. In a method according to claim 3,said semiconductor film is crystallized in said heating step.
 5. Amethod of manufacturing a semiconductor device comprising the stepsof:forming a semiconductor film comprising amorphous silicon over a flatglass substrate; locating the glass substrate on a base having a curvedsurface; heating the glass substrate close to a strain point of theglass substrate thereby crystallizing said amorphous silicon; and thencooling the glass substrate in a way that causes it to flatten, therebyforming the substrate into a flat state.
 6. A method according to claim5 further comprising a step of irradiating a laser beam onto thecrystallized silicon film after the cooling step.
 7. A method ofmanufacturing a semiconductor device comprising the steps of:forming asemiconductor film comprising amorphous silicon over a flat glasssubstrate; locating the glass substrate on a base having a convex curvedsurface; heating the glass substrate at a temperature close to a strainpoint of the glass substrate thereby forming the substrate and theamorphous silicon film into a convex curved shape; and cooling the glasssubstrate in a way that causes it to flatten, thereby forming thesubstrate into a flat state.
 8. A method according to claim 7 furthercomprising a step of irradiating a laser beam onto the silicon filmafter the cooling step.
 9. A method of manufacturing a semiconductordevice comprising the steps of:forming a semiconductor film comprisingamorphous silicon formed over a flat glass substrate; locating the glasssubstrate on a base having a convex U-shaped curved surface; heating theglass substrate at a temperature close to a strain point of the glasssubstrate thereby crystallizing said amorphous silicon film; and thencooling the glass substrate in a way that causes it to flatten, therebyforming the substrate into a flat state.
 10. A method according to claim9 further comprising a step of irradiating a laser beam onto thecrystallized silicon film after the cooling step.
 11. A method ofmanufacturing a semiconductor device comprising the steps of:forming asemiconductor film comprising amorphous silicon over a flat glasssubstrate; locating the glass substrate on a base having a convexU-shaped curve surface; heating the glass substrate at a temperatureclose to a strain point of the glass substrate; and then cooling theglass substrate in a way that causes it to flatten, thereby forming thesubstrate into a flat state.
 12. A method according to claim 11 furthercomprising a step of irradiating a laser beam onto the silicon filmafter the cooling step.
 13. A method of manufacturing a semiconductordevice comprising the steps of:forming a semiconductor film comprisingamorphous silicon over a flat glass substrate; locating the glasssubstrate along a base having a convex U-shaped curved surface; heatingsaid substrate at a temperature close to strain point of the glasssubstrate thereby forming said substrate into a convex curved shape,thereby crystallizing said amorphous silicon; irradiating a laser beamonto the crystallized silicon film while maintaining the glass substrateat a temperature between a room temperature and 70% of a strain point ofthe glass substrate; and then cooling the glass substrate in a way thatcauses it to flatten, thereby forming the substrate into a flat state.14. A method of manufacturing a semiconductor device comprising thesteps of:forming a semiconductor film comprising amorphous silicon overa flat glass substrate; locating the glass substrate on which anamorphous silicon film has been formed, along a base having a convexU-shaped curved surface; heating said substrate at a temperature closeto a strain point of the glass substrate thereby forming said substrateinto a convex curved shape; irradiating a laser beam onto the amorphoussilicon film while maintaining the glass substrate at a temperaturebetween a room temperature and 70% of a strain point of the glasssubstrate; and then cooling the glass substrate.
 15. A method ofmanufacturing a semiconductor device comprising the steps of:forming asemiconductor film comprising amorphous silicon over a flat substrate;locating the glass substrate along a base having a convex U-shapedcurved surface; heating the glass substrate thereby crystallizing theamorphous silicon film; warping the glass substrate along the base byholding down edges of the glass substrate to the base; maintaining theglass substrate at a temperature between a room temperature and 70% of astrain point of the glass substrate; cooling the glass substrate in away that causes it to flatten, thereby forming the substrate into a flatstate; and then irradiating a laser beam onto the crystallized siliconfilm after the maintaining step.
 16. A method of manufacturing asemiconductor device comprising the steps of:forming a semiconductorfilm comprising amorphous silicon over a flat glass substrate; locatingthe glass substrate on a base having a convex U-shaped curved surface;heating the glass substrate; warping the glass substrate along the baseby holding down edges of the glass substrate to the base; maintaining atemperature of from a room temperature to 70% of a strain point of theglass substrate while warping the glass substrate; irradiating a laserbeam onto the amorphous silicon film over the warped glass substrate;cooling the substrate after the irradiating step in a way that causes itto flatten.
 17. A method of manufacturing a semiconductor devicecomprising the steps of:forming a semiconductor film comprisingamorphous silicon over a flat glass substrate; mounting a glasssubstrate on a convex curved surface; heating the substrate therebyforming the substrate into a convex curved shape; flattening the glasssubstrate by cooling; and laser annealing the surface of the glasssubstrate by irradiating a linear laser beam while scanning the linearlaser beam.
 18. A method of manufacturing a semiconductor devicecomprising the steps of:forming a semiconductor film comprisingamorphous silicon over a glass substrate by heating to obtain acrystalline silicon film; mounting the glass substrate on a convexcurved stage; heating the glass substrate and the amorphous silicon filmthereby crystallizing the amorphous silicon film; flattening the glasssubstrate by cooling; and laser annealing the crystalline silicon filmby irradiating a linear laser beam while scanning the linear beam.
 19. Amethod of manufacturing a semiconductor device comprising the stepsof:forming a semiconductor film comprising amorphous silicon over aglass substrate; mounting the glass substrate on a convex curved stage;heating the glass substrate and the amorphous silicon film therebycrystallizing the amorphous silicon film; cooling the substrate in a waythat flattens the substrate; mounting the glass substrate on a stagehaving a flat surface in such a manner that a lower surface of the glasssubstrate is in contact with the flat surface of the stage; and laserannealing the crystalline silicon film by irradiating a linear laserbeam while scanning the linear beam.
 20. A method of manufacturing asemiconductor device comprising the steps of:forming a semiconductorfilm comprising amorphous silicon film over a glass substrate; mountingthe glass substrate on a convex curved stage; heating the glasssubstrate and the amorphous silicon film thereby crystallizing theamorphous silicon film; cooling the substrate in a way that flattens thesubstrate; mounting the glass substrate on a stage having a flat surfacein such a manner that a lower surface of the glass substrate is suckedwith the flat surface of the stage under vapor; and laser annealing thecrystalline silicon film by irradiating a linear laser beam whilescanning the linear beam.
 21. A method of manufacturing a semiconductordevice comprising the steps of:forming a semiconductor film comprisingamorphous silicon film over a glass substrate; mounting the glasssubstrate on a convex curved stage; heating the glass substrate and theamorphous silicon film thereby crystallizing the amorphous silicon film;cooling the substrate in a way that flattens the substrate; mounting theglass substrate on a stage having a flat surface in such a manner that alower surface of the glass substrate is in close contact with the flatsurface of the stage by pressing an upper surface of the glasssubstrate; and laser annealing the crystalline silicon film byirradiating a linear laser beam while scanning the linear beam.